1. Field of the Invention
The present invention relates to a radiation imaging technique which prevents an excessive increase in the exposure amount of a patient.
2. Description of the Related Art
X-ray image diagnosis using X-rays has become indispensable to modern medical services. X-ray image diagnosis has also been generally used for X-ray interpretation using still images. Currently, however, the application of X-ray image diagnosis has extended to examination and surgery support using X-ray fluoroscopy.
In particular, interventional radiology (IVR) has been executed, which allows a doctor to perform treatment by using a catheter inserted in a blood vessel while fluoroscoping the inside of a patient (subject) using X-rays. Executing interventional radiology (IVR) makes it possible to perform surgery on a portion of the body on which it has been in the past been difficult to perform a surgical operation, e.g., the heart or brain.
X-rays can be very useful as described above. On the other hand, it is inevitable that patients are exposed to X-rays. It is therefore necessary to perform diagnosis and surgery support by X-rays with consideration of the risks of associated to exposure. In IVR described above, in particular, X-ray exposure is intermittently executed while the person who performs the surgery (to be referred to as an “operator” hereinafter) performs surgical manipulations (to be referred to as “manipulations” hereinafter). Therefore, the exposure amount in IVR is larger than that in an application using still images. Exposure to X-rays during surgery may cause skin or health hazards such as temporary alopecia. For the above reasons, it has become more important to control the exposure dose.
Conventionally, in general, an operator who performs diagnosis and treatment has manually adjusted the X-ray intensity (the X-ray dose per unit time). The operator has selected a proper X-ray intensity in consideration of a surgical purpose, an examination application, a target region, the built, age, and sex of a patient, the characteristics of an apparatus, and the like. In an operation using fluoroscopy, the operator has manually calculated a dose to make a surgery plan.
There has been also available a technique of automatically controlling the X-ray intensity and image processing to allow an X-ray imaging apparatus to automatically output images suitable for diagnosis and treatment and X-ray interpretation. For example, automatic exposure control (AEC) is used to adjust exposure conditions and image processing parameters so as to always obtain constant image quality on the basis of recorded image information. This makes it possible to always display a fluoroscopic image with constant image quality without requiring the operator during surgery to readjust exposure conditions and image processing parameters as needed.
Automatic exposure control (AEC) described above, however, is just a function of keeping the image quality selected by the operator constant, but is not a function of determining the X-ray intensity itself. In determining the X-ray intensity, it is necessary to consider factors which are difficult to quantify, e.g., the operator's preference with regard to image quality and any applicable examination and treatment policy, as well as the object and type of examination, patient attributes, apparatus configurations, and the like. It is therefore difficult for an X-ray imaging apparatus to determine the X-ray intensity by automatic control only, and hence it is necessary to carry out adjustment of the X-ray intensity manually.
Conventionally, as a warning unit for excessive emission of X-rays from an X-ray imaging apparatus, there has been available a unit for generating a warning or stopping exposure when the continuous exposure reaches a predetermined time, at the time of X-ray fluoroscopy, regardless of the X-ray intensity.
There has also been available a method which allows an operator to set a desired X-ray intensity more safely with a simple operation. There has been generally used an apparatus which selects a preset X-ray intensity by selecting an imaging region. There is also a known method of automatically setting an optimal X-ray intensity on the basis of external information associated with an examination request (for example, Japanese Patent Laid-Open No. 2002-200062).
The first problem is that the conventional scheme of generating a warning on the basis of only exposure time regardless of the intensity of X-rays applied has difficulty in suppressing exposure hazards because the warning generated in this manner is irrelevant to the exposure dose.
The second problem is that the scheme of selecting a preset X-ray intensity in accordance with, for example, the selection of an imaging region has a risk of causing excessive emission of X-rays when an operator performs readjustment after selection. In this case, it is difficult to suppress excessive emission of X-rays. Even automatic control needs to accept adjustment by the operator, and hence has a similar problem.
The present invention has been made in consideration of the above problems, and has as its object to provide an X-ray imaging technique of preventing excessive emission of X-rays in manually setting an X-ray intensity and allowing an operator to set a proper X-ray intensity.